Avoid calling Helper() on the happy path.

Helper() in the standard Go runtime fetches a stack trace from the
runtime, so is slow for calls that are made many times.

Helper() only makes a difference if the call throws an error, so move
it after the test in straightforward cases.

assert/assertions.go

@@ -496,12 +496,12 @@ func Exactly(t TestingT, expected, actual interface{}, msgAndArgs ...interface{}
 //
 //    assert.NotNil(t, err)
 func NotNil(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
 	if !isNil(object) {
 		return true
 	}
+	if h, ok := t.(tHelper); ok {
+		h.Helper()
+	}
 	return Fail(t, "Expected value not to be nil.", msgAndArgs...)
 }
 
@@ -542,12 +542,12 @@ func isNil(object interface{}) bool {
 //
 //    assert.Nil(t, err)
 func Nil(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
 	if isNil(object) {
 		return true
 	}
+	if h, ok := t.(tHelper); ok {
+		h.Helper()
+	}
 	return Fail(t, fmt.Sprintf("Expected nil, but got: %#v", object), msgAndArgs...)
 }
 
@@ -584,12 +584,11 @@ func isEmpty(object interface{}) bool {
 //
 //  assert.Empty(t, obj)
 func Empty(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
-
 	pass := isEmpty(object)
 	if !pass {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		Fail(t, fmt.Sprintf("Should be empty, but was %v", object), msgAndArgs...)
 	}
 
@@ -604,12 +603,11 @@ func Empty(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
 //    assert.Equal(t, "two", obj[1])
 //  }
 func NotEmpty(t TestingT, object interface{}, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
-
 	pass := !isEmpty(object)
 	if !pass {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		Fail(t, fmt.Sprintf("Should NOT be empty, but was %v", object), msgAndArgs...)
 	}
 
@@ -652,16 +650,10 @@ func Len(t TestingT, object interface{}, length int, msgAndArgs ...interface{})
 //
 //    assert.True(t, myBool)
 func True(t TestingT, value bool, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
-	if h, ok := t.(interface {
-		Helper()
-	}); ok {
-		h.Helper()
-	}
-
 	if !value {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		return Fail(t, "Should be true", msgAndArgs...)
 	}
 
@@ -673,11 +665,10 @@ func True(t TestingT, value bool, msgAndArgs ...interface{}) bool {
 //
 //    assert.False(t, myBool)
 func False(t TestingT, value bool, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
-
 	if value {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		return Fail(t, "Should be false", msgAndArgs...)
 	}
 
@@ -1319,10 +1310,10 @@ func InEpsilonSlice(t TestingT, expected, actual interface{}, epsilon float64, m
 //	   assert.Equal(t, expectedObj, actualObj)
 //   }
 func NoError(t TestingT, err error, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
 	if err != nil {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		return Fail(t, fmt.Sprintf("Received unexpected error:\n%+v", err), msgAndArgs...)
 	}
 
@@ -1336,11 +1327,10 @@ func NoError(t TestingT, err error, msgAndArgs ...interface{}) bool {
 //	   assert.Equal(t, expectedError, err)
 //   }
 func Error(t TestingT, err error, msgAndArgs ...interface{}) bool {
-	if h, ok := t.(tHelper); ok {
-		h.Helper()
-	}
-
 	if err == nil {
+		if h, ok := t.(tHelper); ok {
+			h.Helper()
+		}
 		return Fail(t, "An error is expected but got nil.", msgAndArgs...)
 	}